Canadian Consulting Engineer

Weather Beating: Engineering Outdoor Comfort in Hot Climates

December 1, 2013
By By Duncan A. Phillips, Ph.D., P.Eng., & Ryan Danks. P.Eng., RWDI

In many newer communities in the Middle East and Far East, designing for outdoor thermal comfort is becoming one of the “must-have” considerations during the master planning process.

In many newer communities in the Middle East and Far East, designing for outdoor thermal comfort is becoming one of the “must-have” considerations during the master planning process.

The enthusiasm for ensuring outdoor comfort is coming from developers, governments and regulatory bodies who see outdoor comfort as key to healthy lifestyles, lower automobile use and increased economic viability. Indeed some of the sustainability rating systems (e.g. Estidama of Abu Dhabi, GSAS of Qatar) have points awarded for the outdoor microclimate.

The challenge with maintaining outdoor comfort is that in many of these cities the ambient temperature and humidity conditions are extreme in the summer. In winter, the conditions could be described as idyllic. However, during May through September when daytime temperatures can exceed 45°C at lower humidity levels (e.g. Riyadh), and when temperatures of 35°C exist at relative humidity levels of 70% (e.g. Doha), maintaining outdoor thermal comfort is challenging.

Measuring and scoring outdoor thermal comfort is a necessary starting point. In Canada, we use the systems of wind chill in winter and the humidex in summer to describe the impact of certain conditions on temperatures These are useful measures for a quick reference. The drawback is that neither acknowledges the impact of solar insolation. The warmth of the sun improves comfort in winter, but in summer it can make uncomfortable conditions unbearable.

These measures also do not account for the combined effects of wind and humidity: low humidity levels can make an individual “feel” cooler, but without adequate air movement this effect is reduced. A windy, humid space in the sun can be much more comfortable than a stagnant dry space in shade. The other drawback is that one cannot use humidex and wind chill to rate the beneficial design of an outdoor space.

While other measures of outdoor thermal comfort exist, RWDI prefers to use an indoor measure adapted for outdoor use. We call this SPMV* and calibrated it during work for a project called Masdar in the United Arab Emirates several years ago. Similar to the Predicted Mean Vote (PMV) scale promulgated by ASHRAE, SPMV numerically scores the comfort of a space where a value of 0 represents thermally neutral conditions. Higher positive values indicate warmer conditions, and negative values are cooler.

To compute this metric we first conduct a series of computational fluid dynamics (CFD) simulations to assess the built environment’s influence on local wind conditions for the relevant wind directions found on site. These simulations are combined with a proprietary solar analysis engine to allow for an hour-by-hour computation of thermal comfort.

If, therefore, one is able to predict annual wind and solar conditions throughout a master plan, then one can predict the thermal comfort everywhere. Furthermore, if one is able to score the thermal comfort for one configuration, then beneficial changes to massing, or other interventions, can be assessed to generate a climate-aware master plan. RWDI has used city orientations, massing adjustments, building topologies (e.g. colonnades), shading devices and other strategies to passively manipulate the climate within cities. The relative performance of one adjustment over another can be weighed against other parameters that are of interest. Strategies, including material choice and selective wind enhancements, can also lead to improvements in comfort.

While the analysis of thermal comfort is one aspect of city planning, other important factors such as the energy demand of buildings, daylight availability and natural ventilation potential can also be assessed using similar tools during the master planning process. The positioning of buildings relative to one another, their heights and massing, can be used to augment each of these parameters to arrive at a “sweet spot” for all of these design aspects.

Image 1 (p. 27) shows a master planning project in Doha, Qatar called Hamad International Airport (HIA) Airport City, developed for the Airport Steering Committee. It is intended to be a link between the new airport in Doha and the surrounding community, which will be mixed-use with integrated business, logistics and residences. Public transport, low energy demands and a comfortable microclimate were key elements of the planning.

Image 2 shows the solar impacts within the community through a measure called the “sky view factor.” The yellow, orange through black colouring quantifies the degree to which each point in the master plan is exposed to the sky and thus solar radiation. The building massing, canopies and trees were included in the analysis.

Image 3 shows the average annual wind speed within the city. This is calculated by (a) simulating winds from all relevant directions, and then (b) weighting the results by speed and frequency to generate the annual prediction. The hourly solar calculations, coupled with the wind predictions, can be combined to provide a prediction of thermal comfort, using the climate record containing temperature and relative humidity too. A sample is provided in Image 4. In this case, the calculation is presented for spring afternoons, but any period of the year or statistic can be generated.

Having generated estimates of thermal comfort, the beneficial impact of different adjustments can be weighed. Image 5 shows the before and after consequences of adding a wind tower to ventilate a city courtyard. This is one means to improve thermal comfort.

There was a time when cities would be designed over the course of decades and centuries. During that time, architects and planners could tweak the design based on observations of performance. Today, we move faster. The ability to analyze potential performance in advance and find strategies to improve conditions can lead to lower energy demand, more comfortable cities, and better spaces for people.cce

Duncan A. Phillips, Ph.D., P.Eng. is a principal with RWDI of Guelph, Ont.. He consults on projects that involve optimizing the interaction of buildings within various climates. Ryan Danks. P.Eng. is a research and development engineer with RWDI. He specializes in creating tools and methodologies to simulate the interaction between the built and natural environments.


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